Surgical drill for use with a computer assisted surgery system

ABSTRACT

A surgical drill for use with a computer assisted surgery system. In a first embodiment, at least three localizing emitters are mounted on and integral with each side of the drill housing, the emitters on each side arranged such that they are visible to a localizing device when that side is facing the localizing device. The emitters on each side are spaced sufficiently apart from one another to provide accurate pose information. In a second embodiment, the emitters are mounted on both sides of a two-sided fin assembly attached to the housing.

REFERENCE TO PRIOR APPLICATION

[0001] This is a divisional of co-pending application Ser. No.09/878,588, filed Jun. 11, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a surgical drill for use with acomputer assisted surgery system.

[0004] 2. Description of the Related Art

[0005] Computer assisted surgery systems have been developed to aid asurgeon in more accurately positioning instruments during a surgicalprocedure. These systems have been described for use with CT images,fluoroscopic images, and images derived from other modalities as well aswithout images or in conjunction with a surgical robot. When usingimages of a patient, they are often referred to as image guided surgerysystems. These systems typically use a localizing device to tracksurgical instruments in real time. One commonly used localizing deviceis an optical localizer that employs a stereoscopic camera system toview infrared light emitters or reflectors that are placed on thesurgical instruments. Other localizing devices may use electromagneticradiation or a passive manipulator arm with position encoders at thejoints. The computer assisted surgery system then uses the informationregarding the instrument's position to superimpose a representation ofthe instrument over the images or to generate some other graphic oralphanumeric display.

[0006] Commonly, surgical procedures performed with computer assistedsurgery systems involve the insertion into the patient of an elongateinstrument such as a drill bit or a probe. In these cases thecomputer-assistance may consist of superimposing a line representing thetrajectory of the drill or probe over images of the patient. Manysystems additionally provide for a means for determining the location ofthe tip of the probe or drill bit. By comparing this tip location to thelocation of a target feature in the body, the software can calculate thedistance remaining in the trajectory. This information can be displayedas a bar graph as taught by U.S. Pat. Nos. 5,251,127 and 6,006,127 or asa numeric value or other graphic as taught by U.S. Pat. No. 5,638,819.However, these approaches are not suitable for orthopaedic procedures,where the surgeon does not typically record a target point prior to theprocedure.

[0007] Another problem with the approach of tracking the drill itself isthat the drill bit may bend. This can cause the trajectory displayed bythe computer assisted surgery system to be significantly different thanthe actual path of the drill bit. Bending of the drill bit is mostlikely to occur at a location between the drill and the body part,either as a result of deflection of the drill bit tip upon entering thebody part or because of lateral forces placed on the drill by thesurgeon. One way to alleviate this problem is to use a tool guide thatallows the trajectory of the drill bit or probe to be measured as itenters the body part. Such tool guides are common in orthopaedic surgeryand neurosurgery. Computer assisted versions of tool guides aredescribed in U.S. Pat. Nos. 5,517,900 and 5,904,691, among others. Witha tool guide placed flush against the bone the opportunity for drill bitbending is minimized and the trajectory displayed by the computer moreclosely matches the actual trajectory. However, tracking just the toolguide provides information regarding only the trajectory of the drillbit and not its progress into the body.

[0008] The present invention provides the surgeon with improvedinformation for more accurately inserting a drill bit or other elongateobject into the body, when used with a drill guide and a computerassisted surgery system.

SUMMARY OF THE INVENTION

[0009] Accordingly, one objective of the present invention is to providea computer assisted surgery system for accurately positioning a drillbit into a body part.

[0010] Another objective of the invention is to provide a technique andapparatus for calculating and graphically displaying the pose of a drillbit attached to a drill and passed through a drill guide by determiningthe poses of the drill and drill guide. The invention has the advantageof accurately displaying the orientation and location of the drill bit.

[0011] Still another objective of the invention is to provide atechnique and apparatus for reporting to the surgeon misalignment,malposition, or excessive bending of the drill bit relative to the drillguide.

[0012] Yet another objective of the invention is to provide a techniqueand apparatus for accurately calculating and graphically displaying thepose of a drill bit attached to a drill with only a single visiblelocalizing emitter and passed through a drill guide.

[0013] Still another object of the invention is to provide a surgicaldrill, with three or more localizing emitters integrated into the drillhousing, whose pose can readily be determined with the use of alocalizer.

[0014] These and other objects of the present invention are achieved bythe use of a computer assisted surgery system, including a drill and adrill guide, each outfitted with three or more localizing emitters, andan optical localizer for measuring the pose of the drill and drillguide. A drill bit is attached to the drill and the location of the tipof the drill determined during a calibration step in which the tip ofthe drill is placed against a dimple in the drill guide and the poses ofthe two instruments recorded. During drilling, the drill bit is insertedthrough the bore of the drill guide and the poses of the drill guide anddrill are recorded in real time. The pose of the drill bit is calculatedfrom the orientation of the drill guide and the location of the drill. Agraphic representation of the drill bit is then displayed on the monitorscreen in an appropriate position relative to the stored images or otherinstruments.

[0015] In an alternative embodiment of the drill, the three or morelocalizing emitters are integrated into the drill housing and are spacedwidely apart to facilitate accurate pose determination.

[0016] In an alternative embodiment of the computer assisted surgerysystem, the drill may be outfitted with one or more localizing emitters.In the calibration step, the distance between the drill's localizingemitter and the tip of the drill is determined. During drilling, thepose of the drill bit is calculated from the orientation of the drillguide and the location of the drill's localizing emitters, prior todisplay of its representation on the monitor screen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a perspective view of a drill and drill guide withattached housings containing localizer emitters according to the presentinvention.

[0018]FIG. 2 is a perspective view of a drill with localizer emittersintegrated into the drill housing.

[0019]FIG. 3 is a perspective view of the drill of FIG. 1 and a drillguide in position for calibration.

[0020]FIG. 4 is an example of a screen display of two images andsuperimposed instrument representations.

[0021]FIG. 5 is a perspective view of the drill and drill guide inposition for use in drilling.

[0022]FIG. 6 is a perspective view of a drill with a single localizeremitter integrated into the drill housing and a drill guide in positionfor calibration.

[0023]FIG. 7 is a perspective view of the drill of FIG. 6 with anintegrated localizer emitter and the drill guide in position for use indrilling.

[0024]FIG. 8 is a perspective view of the drill and drill guide of FIG.5 in position for use in drilling.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention is preferably operated in conjunction withan image guided surgery system such as is disclosed in U.S. patentapplication Ser. No. 09/248,133 entitled “Computer Assisted TargetingDevice for Use in Orthopaedic Surgery”. In the preferred embodiment,this image guided system comprises a computer, a display monitor, anoptical localizing device, and surgical instruments outfitted withinfrared LEDs as localizing emitters viewable by the optical localizer.During surgery the system functions by acquiring x-ray images with aC-arm fluoroscope of the involved body part. An optical localizer iscapable of measuring the location of surgical instruments outfitted witha single emitter and the pose (location and orientation) of surgicalinstruments with three or more emitters. The system then superimposes onthe images graphic representations of the instruments at their currentpositions. This allows the surgeon to view, in real time, the positionof the instruments with respect to an imaged body part or with respectto other instruments.

[0026] In the preferred embodiment of the present invention, thesurgical instruments are a drill guide and a drill with an attacheddrill bit. As shown in FIG. 1, the drill guide 110 comprises a bore orchannel portion 114 through which may pass the drill bit 105. The boreportion 114 has a proximal end (drill bit entrance) 107 and a distal end(drill bit exit) 108. The bore portion of the drill guide functions todirect the drill bit, which passes through it on a trajectory thatmatches that of the bore. The drill guide 110 further comprises a finassembly 112 that houses eight localizing emitters 111, four on eitherface of the fin assembly 112, in known locations relative to the boreportion 114. In an alternative embodiment, fewer localizing emitters maybe used as long as there are at least three emitters visible to thelocalizing device during use. The localizing emitters may comprisereflectors, spheres, visible spectrum emitters, or any other suitabledevices capable of being accurately located by an optical localizer.Alternatively, other localizing devices may be employed includingacoustic localizers, electromagnetic localizers, or passivemanipulators. Optionally, a handle 104 may be attached to the drillguide 110 to provide a convenient means for its manual positioning.

[0027] The drill guide 110 preferably also comprises a calibrationportion, such as a dimple 113 or other indentation on a surface of thedrill guide 110 capable of accepting a tip 106 of a drill bit 105 orother elongate object in a repeatable and accurate manner. The dimple113 is located in a known relationship to the localizing emitters 111.Any other feature of the drill guide 110 capable of constraining the tipof an elongate object in a known location relative to the drill guide'slocalizing emitters 101 may be used without departing from the presentinvention.

[0028] In the preferred embodiment of the present invention, the drill100 comprises a drill housing 115 and a rotating chuck 116 capable offirmly gripping a drill bit 105. The drill 100 has an axis of rotationabout which the chuck 116 rotates relative to the drill housing 115. Forcalculation purposes, a reference frame may be assigned with one axiscoincident with the axis of rotation and the origin assigned at thepoint of attachment of the drill bit to the drill. The drill 100 furthercomprises an attached fin assembly 102 that houses eight localizingemitters 101, four on either face, in known locations relative to thedrill's axis of rotation, such that they define a drill emittercoordinate frame, D.

[0029] In an alternative embodiment shown in FIG. 2, the drill 100comprises six localizing emitters 101, three localizing emitters 101 oneach side of the drill housing 115, which are arranged in a novel andcharacteristic manner and which define a drill emitter coordinate frame,D. A novel aspect of this invention is that the emitters 101 on thedrill 100 are integral to the standard drill housing 115 and that therequisite wide spacing of the emitters 101 is achieved by placing themat the ends of the drill body 117, 118 and the bottom area of the handle119, areas which are readily visible to the localizing device and arenot likely to be obstructed by the surgeon's hand. Further, the spacingof the emitters 101 is wider than would be achieved by a typicallydimensioned fin 102, and thus greater pose accuracy is possible.Alternatively, any arrangement of emitters that allows the localizer todetermine the pose of the drill emitter coordinate frame with sufficientaccuracy may be used without departing from the instant invention.

[0030] The emitters 101 are activated by signals supplied by a cable 103from the optical localizer. This cable 103 may share functions with apower supply or other cable. Alternatively, the emitters 101 may beactivated by a cableless system in which the power is supplied by abattery in the drill 100 or a separate battery. In an alternativeembodiment, the localizing emitters may comprise reflectors, spheres,visible spectrum emitters, or any other suitable devices capable ofbeing identified and accurately localized by an optical localizer.Alternatively, other localizing devices may be employed includingacoustic localizers, electromagnetic localizers, or passivemanipulators.

[0031] In the preferred embodiment, the computer assisted surgicalsystem displays graphic representations of surgical instruments bymeasuring the pose of the instrument and then simulating the projectionof points of a 3-D model of the instrument at its measured pose onto a2-D plane associated with the C-arm at the time the x-ray images areacquired. For the drill guide, the 3-D model is preferably a vertex andline representation of a tube defining the portion of the drill guidecontaining the bore and a trajectory line representing an extension ofthe bore of the drill guide. For the drill, the 3-D model is a vertexand line representation of the drill bit that is inserted into thedrill's chuck. Alternatively, the graphic representation of aninstrument may be any model that includes information about itsfunctional features from a complex surface or solid rendering of theentire instrument to a simple line trajectory for the drill guide or an“X” to represent the tip of the drill bit. In an alternative embodiment,these models may be projected onto any arbitrarily defined 2-D plane todisplay one instruments relative to another instrument.

[0032] To generate the preferred drill bit representation 202, FIG. 4,the location of the tip 106 of the drill bit 105 must be determined. Asseen in FIG. 3, this is preferably accomplished during a calibrationstep by placing the tip 106 of the drill bit 105 into the dimple 113 onthe drill guide 110. The poses of both instruments 100, 110 are recordedwhen a footswitch is pressed or other triggering means is activated. Thelocation of the tip 106 of the drill bit 105 relative to the drillemitter coordinate frame D can be calculated from the pose of the drillguide 110, the known location of the dimple 113 on the drill guide 110,and the pose of the drill guide coordinate frame, using methods wellknown in the art. The location ^(D)q of the tip 106 of the drill bit 105with respect to the drill emitter coordinate frame D is then stored inthe computer.

[0033] In an alternative embodiment, the calibration step is performedby inserting the drill bit 105 into the bore 114 of the drill guide 110and simultaneously placing the distal end 108 of the drill guide bore114 and the tip 106. of the drill bit 105 against a rigid object, suchas the cortex of a bone. The poses of both instruments 100, 110 arerecorded in response to a triggering means, and the location of the tip106 of the drill bit 105 relative to the drill emitter coordinate frameD is calculated from the pose of the drill guide 110, the known locationof the distal end 108 of the bore 114 of the drill guide 110, and thepose of the drill guide coordinate frame, D, using methods well known inthe art. The location ^(D)q of the tip 106 of the drill bit 105 withrespect to the drill emitter coordinate frame D is then stored in thecomputer.

[0034] Alternatively, the tip 106 of the drill bit 105 may be placed ina dimple on an immovable fixture and the drill rotated around this pointwhile positions of the drill's emitters are sampled, as described inU.S. patent application Ser. No. 09/248,133. The tip 106 of the drillbit 105 is defined as the center of the sphere that is described by thedrill emitters 101 and is found by numerical methods known in the art.Any other method for determining the position of the drill bit tip maybe employed without departing from the instant invention.

[0035] Next, in order to display the drill bit 105, its pose must bedetermined. One suitable pose, ^(O)T_(E), represents the pose of the tip106 of the of the drill bit 105, with respect to the optical localizerwhen the drill bit 105 is straight. This pose can be calculated usingmethods well known in the art from the orientation of the drill's axisof rotation relative to the drill emitter coordinate frame D, the pose,^(O)T_(D), of the drill 105 relative to the localizer, and the location^(D)q of the drill bit tip 106 relative to the drill emitter coordinateframe. Turning to FIG. 4, the drill bit's graphic representation 202 canthen be calculated from the drill bit pose ^(O)T_(E), the drill bit'spreviously stored 3-D model, and a previously determined model of theC-arm imaging chain. The graphic representation of the drill bit 202 issuperimposed on the x-ray images of the body part 204. Alternatively, a2-D model of the drill bit may be generated and superimposed on thex-ray images based on the tool pose. Additionally, a model of the drillguide may be superimposed on the images using similar techniques.

[0036] For an accurate drill bit representation 202 to be displayed inthe foregoing description, the orientation of the drill bit 105 mustcoincide with the axis of rotation of the drill 100. However, as thedrill bit 105 encounters obstacles during insertion into the bone, itmay exhibit some flexion relative to the drill 100, causing the actualtrajectory at the tip 106 to deviate from the drill bit representation202. Therefore, in the preferred embodiment shown in FIG. 5, the effectsof drill bit flexion are minimized by passing the drill bit 105 throughthe bore 114 of the drill guide 110 just prior to entry into the bodypart, significantly reducing the distance over which flexion may occur.In this preferred case, the pose ^(O)T_(E) for the tip 106 of the drillbit 105 is calculated more accurately by combining the orientation ofthe bore 114 of the drill guide 110 with the location of the tip 106 ofthe drill bit 105. The orientation of the bore 114 of the drill guide110 is derived from the known orientation of the bore on the drill guideand the pose of the drill guide as measured by the localizing device.The location ^(O)T_(D) of the tip 106 of the drill bit 105 is derivedfrom the location ^(D)q of the tip 106 relative to the drill emittercoordinate frame D and the pose of the drill as measured by thelocalizing device.

[0037] Additionally, a reference frame can be assigned with an axisparallel to the axis of rotation of the drill 100 and with its originarbitrarily at the point of attachment of the drill bit to the drill.The orientation and location of this reference frame can be comparedwith the orientation and location of the bore of the drill guide 114,and the difference reported to the surgeon by means of text or graphicson the display or by an audible alarm or other signal. An errorcondition can be signaled to the surgeon if the orientations orlocations of the two instruments are not sufficiently coincident. Forexample, a difference in orientation could be caused by excessiveflexion of the drill bit 105 or failure to insert the drill bit 105 intothe bore 114 of the drill guide 110. A difference in locationperpendicular to the axis of rotation of the drill is likely to becaused by failure to insert the drill bit 105 into the bore 114 of thedrill guide 110, while a difference along the axis of rotation is normalduring drilling and is related to depth of insertion of the drill bit105.

[0038] In general, a plurality of tracked instruments may interact toform more accurate or useful graphic representations by combining orcomparing their orientations and locations without departing from theinstant invention. In an alternative embodiment, a tracked cutting guideconstrains the saw blade of a tracked saw and the graphic representationof the saw blade is generated using the plane defined by the cuttingguide, and the position and orientation of the saw.

[0039] In an alternative embodiment, shown in FIG. 6, the drill 100 isoutfitted with a pair of localizing emitters 101 attached to the drillhousing 115. The emitters 101 are attached to either side of the drillhousing 115 such that one emitter 101 is viewable by the localizingdevice during use. Alternatively, the two emitters or reflectors may bemounted on either side of a removable housing that attaches rigidlyanywhere on a drill. In another embodiment, a single reflective spheremay be mounted on the top of the drill or on a part of the drill whereit is viewable by the localizer from either side of the drill.

[0040] In order to generate a graphic representation 202 of the drillbit 105 when only a single emitter 101 is visible on the drill, thedistance, d, between the emitter 101 and the tip 106 of the drill bit105 must be found. This may be calculated during a calibration step asthe difference between the position of the drill emitter 101 and a knownreference point such as a dimple 113 on the drill guide 110 or thedistal end 108 of the bore 114 of the drill guide 110. The surgeonplaces the tip 106 of the drill bit 105 coincident with the dimple 113or other reference point and simultaneously presses a footswitch orother triggering device, causing the pose of the drill guide 110 and thelocation of the drill's emitter 101 to be measured and the differencecalculation to be performed.

[0041] During drilling, as shown in FIG. 7, while the drill bit 105 isinserted into the bore 114 of the drill guide 110, the pose of the drillguide 110 and the location of the drill's emitter 101 are measured bythe localizing device. A coordinate frame F is assigned a position alongthe bore 114 of the drill guide 110, and its pose ^(O)T_(F) iscalculated from the known position of F on the drill guide 110 and fromthe pose of the drill guide 110 as measured by the localizer. Thelocation ^(F)p of the drill emitter 101 can then be calculated relativeto F. Using this and the stored value of the distance d between the tip106 of the drill bit 105 and the drill emitter 101, the pose of the tip106 with respect to the localizer can be calculated by the formula:${{}_{}^{}{}_{}^{}} = {{{}_{}^{}{}_{}^{}} \cdot \begin{bmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & 1 & l \\0 & 0 & 0 & 1\end{bmatrix}}$

[0042] where l is the distance between the tip 106 of the drill bit 105and the origin of the tool coordinate frame. This is calculated from theequation:

l={square root}{square root over (d²−^(F)p_(x) ²−^(F)p_(y) ²)}−| ^(F) p_(z)|

[0043] where d is the previously determined distance between the drillemitter and tip 106 of the drill bit 105, and ^(F)p_(x), ^(F)p_(y), and^(F)p_(z)are the Cartesian coordinates of ^(F)p in coordinate frame F.

[0044] In the preferred embodiment, shown in FIG. 4, the system beginscalculating and displaying the position of the drill bit representation202 once the footswitch is pressed to perform the calibration step. Asecond press of the footswitch turns off the calculation and display ofthe position of the drill bit representation 202. While the drill bitdisplay is active, the system also displays on the monitor a numericalvalue 203 representing the distance the tip 106 of the drill bit 105protrudes past a reference point. Preferably, as shown in FIG. 8, thereference point is the distal end 108 of the bore 114 of the drill guide110. The depth value, m, represents the distance between the drill bittip 106, at q, and the reference point, at r, on the drill guide 110.The position, ^(O)q, of the tip 106 of the drill bit 105 is known fromthe calculation of the pose ^(O)T_(E) of the drill bit coordinate frameE (for either a single emitter drill or multiple emitter drill). Theposition, ^(O)r, of the reference point, is known from the pose of thedrill guide 110 as measured by the localizer and the position of point ron the drill guide 110. The depth, m, is calculated as the magnitude ofthe difference between ^(O)q and ^(O)r.

[0045] Alternatively, the depth may be reported in other formatsincluding a bar graph representation.

[0046] The distance traveled by the drill bit 105 may also be calculatedand reported for any arbitrary starting and ending points. The startingpoint may be assigned to be the location of the drill bit tip 106 whenthe footswitch is pressed a first time and the ending point assigned tobe the location when the footswitch is pressed a second time. This canallow, for example, measurement from the lateral cortex to the medialcortex of a bone, regardless of drill guide placement.

[0047] Alternatively, the signals to indicate the enabling or disablingof the calculation and display of the drill bit representation, or ofthe assignment of a starting or ending point for distance calculation,may be generated in any manner without departing from the instantinvention. For example, these signals may be generated by the pressingof a trigger on the instrument, the activation of a voice activatedswitch, or the sensing of the starting or stopping of the drill motor.

[0048] Alternatively, a graphic may be employed that displays therepresentation of the drill bit relative to another surgical instrumentor instruments. In this case, the superposition of the instruments onimage data is optional and the representation of the drill bit isdisplayed relative to the representation of another instrument orinstruments or relative to the graphics window (where the frame ofreference of the graphics window is determined by the pose of the otherinstrument or instruments).

[0049] The herein described invention may be applied to surgeries,surgical instruments and situations beyond those outlined in thisdescription. Other modifications and alternative embodiments of theinvention are contemplated which do not depart from the spirit and scopeof the invention as defined by the foregoing teachings and appendedclaims. It is intended that the claims cover all such modifications thatfall within their scope.

We claim as our invention:
 1. A surgical drill for use with a computerassisted surgery system having a localizing device, the drillcomprising: a housing having a front end; a rotatable chuck mounted onthe front end of the housing, the chuck defining an axis of rotation; afin assembly attached to the housing, the fin assembly having two sides;and at least four localizing emitters mounted on each side of the finassembly in known locations relative to the axis of rotation, theemitters on each side arranged such that they are visible to thelocalizing device when that side of the fin assembly is facing thelocalizing device.
 2. A surgical drill for use with a computer assistedsurgery system having a localizing device, the drill having two sides,the drill comprising: an elongated housing having opposite ends; ahandle attached to the housing and having a bottom area away from thehousing; and at least three localizing emitters mounted on and integralwith each side of the drill, the emitters on each side arranged suchthat they are visible to the localizing device when that side is facingthe localizing device, the emitters on each side spaced sufficientlyapart from one another to provide accurate pose information.
 3. Thesurgical drill of claim 2 wherein, on each side of the drill, twoemitters are mounted on opposite ends of the housing and one emitter ismounted on the bottom area of the handle.